1. Field of the Invention
This invention relates to sensors composed of ceramic and metallic parts which are subject to large variations in temperature. It has particular application to ion conductive solid electrolyte gas sensors used in detecting the oxygen concentration in the exhaust gases of an internal combustion engine.
2. Prior Art
U.S. Pat. No. 4,111,778 assigned to the assignee of the present invention, and the contents of which are incorporated by reference herein, discloses an oxygen gas sensor of the type in which one surface of an ion conductive solid electrolyte, such as zirconium dioxide, is exposed to a reference concentration of oxygen and an opposed surface is exposed to the oxygen concentration to be sensed. A difference in oxygen concentration on the two sides of the sensor element generates an electrical potential between the two surfaces which is representative of this difference in oxygen concentration. For use in analyzing or controlling the fuel/air ratio of a combustion engine gas mixture, the surface of the solid electrolyte can be coated with a catalyst so that the sensor will produce an electrical step function as the air/fuel ratio goes through stoichiometric from a rich mixture to a lean mixture.
As in many solid electrolyte sensors, the zirconium dioxide element in U.S. Pat. No. 4,111,778 is shaped in the form of a tube closed at one end and open at the other. The tube is mounted in a metallic shell which screws into the exhaust system of an internal combustion engine with the closed end of the tube disposed in the gas stream to be analyzed and the open end exposed to ambient air. A ventilated, metallic sleeve is provided over the open end of the tube to protect it from water and solid contaminants. Likewise, the closed end of the tube may be covered by a fluted, cup-shaped, metallic shield which protects the solid electrolyte from particulates in the gas stream.
Both the inside and outside surfaces of the solid electrolyte tube are coated with a porous layer of platinum or palladium which serves as a catalyst for the gases in contact therewith and as an electrode for the sensor. The coating on the outside of the tube is electrically connected to the metallic shell which serves as a ground terminal for the sensor. The coating on the inside of the tube is in electrical contact with a stud seated in a counterbore in the tube. A helical compression spring mounted on the stud bears against a metallic terminal axially retained by an insulator mounted in the end of the ventilated sleeve covering the open end of the tube to complete the electrical circuit between the interior surface of the tube and the terminal.
In addition to completing the electrical circuit for the internal surface of the sensing tube, the helical compression spring accommodates for movement of sensor parts with respect to each other resulting from thermal expansion. This is important because a sensor used in detecting the oxygen concentration in the exhaust gases of an internal combustion engine may be exposed to operating temperatures of from 300.degree. up to 1000.degree. Celsius. Furthermore, the temperature difference between the electrical terminal and the sensing tube could be very high when it is considered that the temperature of the sensor could be sub-zero before starting of the engine. If a uniformly wound compression spring with open coils is seated directly in the counterbore in the sensing element, the high operating temperatures will cause the portion of the spring in the counterbore area to relax, thereby impairing the electrical contact between the terminal and the internal surface of the sensing tube. The stud was used in the sensor disclosed in U.S. Pat. No. 4,111,778 to raise the spring out of the hotter area in the counterbore in order to alleviate this problem. The stud, however, is an expensive item to manufacture which, of course, raises the cost of the sensor.
Thus, it is a primary object of this invention to provide a solid electrolyte gas sensor which is reliable over the full range of operating temperatures to which it is exposed and can be produced as economically as possible.
Specifically, it is an object of this invention to provide such a sensor which accommodates for thermal expansion of the various components of the sensor while maintaining electrical continuity in the sensor circuits over the full thermal operating range.
More specifically, it is an object of this invention to provide a helical compression spring for such sensors which will maintain at least a minimum force against components of the internal sensor electrical circuit over the full thermal operating range.